(1) Field of the Invention
This invention relates to an improvement in a power transmission, more particularly to an improvement in a power transmission which makes use of one or more centrifugal hydraulic-pressure compensating clutches which can be suitably incorporated in an automatic transmission.
(2) Description of the Prior Art
An automatic transmission for a vehicle engine is generally constructed as a combination of a torque converter and a change speed gear assembly. The change speed gear assembly in turn includes a plurality of hydraulic clutches so as to perform the connection or disconnection between an associated drive element and driven elements by either supplying or discharging hydraulic fluid to or from their corresponding hydraulic cylinders. A desired shift position can be selected by suitably engaging or releasing these clutches. In other words, the shifting can be achieved by selectively connecting or disconnecting plural frictional engaging elements. Many of such frictional engaging elements comprise wet-type multiple disk clutches. The engagement or release of each of these clutches can be achieved by supplying or discharging a hydraulic fluid to or from a hydraulic chamber, which is surrounded by its piston and its cylinder holding the piston therein, and hence causing the piston to advance or retreat.
A description will next be made of the advancing and retreating motion of a piston upon supply and discharge of a hydraulic pressure to and from its associated hydraulic chamber. Where a combination of frictional elements which are selectively engaged and released are both rotary elements, the cylinder rotates at the same speed as either one of the rotary elements and both the supplied hydraulic pressure and centrifugal hydraulic pressure (the pressure produced by the centrifugal force applied to the remaining hydraulic fluid as a result of rotation) are therefore exerted upon the piston.
Even when the supplied hydraulic pressure is reduced to 0 kg/cm.sup.2 in order to release the frictional elements, a piston-pushing force still remains owing to the existence of the centrifugal hydraulic pressure. In some instances, the clutch may not be fully released in a short period of time by the force of its return spring.
Further, a centrifugal pumping action may also take place as a result of the rotation of the frictional elements and the hydraulic fluid may be continuously supplied.
The clutch may be maintained in a semi-engaged state for the reasons mentioned above, leading to such inconvenience that the shifting time may become longer or the clutch may be burnt out.
As methods for solving these inconveniences, the following power transmissions have been proposed:
(1) As illustrated in FIG. 3, check valves 23 are provided outside the cylindrical walls of pistons 08,016 (or cylinders 09,017) of the clutch-operating actuator.
(2) As depicted in FIG. 4, a centrifugal hydraulic fluid chamber 012, to which hydraulic fluid of approximately atmospheric pressure is supplied, is formed, on one side of a first piston 08 of the clutch-operating actuator, the one side being opposite to a working fluid chamber 010 relative to the piston 08, by means of a partition 011 and the first piston 08 so as to compensate centrifugal hydraulic pressures to be developed in the working fluid chamber 010.
The following problems however arise when the inconvenience caused by centrifugal hydraulic pressures are tried to be overcome by the above-described methods (1) or (2). (1) With respect to the structure shown in FIG. 3:
(a) The hydraulic fluid remaining outside the check valves 23 is not discharged so that the centrifugal hydraulic pressure cannot be completely eliminated.
(b) When the hydraulic fluid is discharged, air enters the working fluid chambers 010 and 018. Accordingly, a long period of time is required to fill up the working fluid chambers 010 and 018 with the hydraulic fluid so as to bring the clutches 05 and 013 again into their engaged state. A long period of time is also required to increase the hydraulic pressure to such a level that the force applied to the balls 23a by the supplied hydraulic pressure overcomes the centrifugal forces exerted upon the balls 23a so that the check valves 23 are then closed. As a result, the shifting time becomes longer.
(c) When driving at a high rate of speed, the hydraulic fluid is drawn by the centrifugal pumping action, and the clutches 05 and 013 cannot be released unless air is fed into the working fluid chambers 010 and 018. (2) With respect to the structure illustrated in FIG. 4:
By filling the centrifugal pressure chamber 012 formed by the piston 08 and partition 011 with a hydraulic oil (for example, by introducing a usual forced lubricating oil), a centrifugal hydraulic pressure P.sub.1 applied from the side of a retainer 02 to the piston 08 by the hydraulic fluid contained in the working fluid chamber 010 becomes equal to another centrifugal hydraulic pressure P.sub.2 applied from the side of the partition 011 to the piston 08 by the hydraulic fluid contained in the centrifugal pressure chamber 012 so that the centrifugal hydraulic pressure applied to the piston 08 is compensated or balanced. Although the inconvenience caused by such centrifugal hydraulic pressures is solved by the above method, it is necessary to define the centrifugal pressure chamber 012 and to provide the partition 011 for defining the centrifugal pressure chamber 012. Correspondingly, the axial dimension of the actuator for the clutch 05 becomes longer and the space required for the installation of the clutch 05, that is, the overall length of the clutch 05, increases. This is extremely disadvantageous from the viewpoint of installation space especially when two or more clutches are provided or required.